Macrophages (Mφ) contribute to the resolution of early inflammation by recognizing and ingesting apoptotic polymorphonuclear neutrophils (PMN). In addition, experiments reported here demonstrated that Mφ can actively induce PMN apoptosis. Coculture of cells from 2-or 5-day-old wounds in rats, or of Mφ purified from such preparations, with PMN-rich wound cell populations obtained 1 day after wounding increased PMN apoptosis by> 3-fold. Neither resident-nor Proprionibacterium acnes-elicited peritoneal Mφ-induced PMN apoptosis. Apoptosis was not mediated by a soluble factor and required E: T contact. Fixed wound-Mφ and membrane isolates from viable Mφ were as effective as intact cells in inducing PMN apoptosis. Mφ-induced apoptosis was inhibited by peptide Arg-Gly-Asp-Ser, anti-β 3 (CD61) Ab, CD36 peptide, or anti-TNF-α Ab. Soluble TNF-α did not induce PMN apoptosis. In additional studies, K562 cells (negative for β 3, TNF-α, and Fas ligand) transfected to express either α v β 3 integrin, an uncleavable membrane form of TNF-α, or both were used in cocultures with wound PMN. Only the double transfectants were able to induce PMN apoptosis, an effect inhibited by anti-β 3 (CD61) or anti-TNF-α Abs. These results demonstrate that wound Mφ induce PMN apoptosis through a constitutive effector mechanism requiring both intercellular binding through integrin-ligand interactions and membrane-bound TNF-α.